Hematoporphyrin IX (8,13-bis(1-hydroxvethyl)-3,7.12.17-tetramethyl-21H-23H-porphine-2,18-dipropionic acid) and protoporphyrin IX (8,13-divinyl-3,7.12.17-tetramethyl-21H-23H-porphine-2. 18-dipropionic acid) were efficiently immobilized on a cellulose.,titanium (IV) oxide composite fiber surface by the reaction of the porphyrin COOH groups with TO,. presumably by forming the -COO-Ti chemical bond. Furthermore, Co(II) was incorporated into the porphyrin ring. with this reaction being followed by UV-vis spectra in the solid state and confirmed by, the change of the absorption bands due to a local symmetry change from D-2h to D-4h upon metallation of the porphyrin ring. Electrochemical studies by using cyclic voltammetry and chronoamperometry techniques, showed that the immobilized complexes catalyzed O-2 reduction at - 0.18 V for hematoporphyrin and - 0.20 V for protoporphyrin in 1 mol l(-1) KCl solution at pH 6. The cathodic current peak intensities plotted against O-2 concentrations in the range from 0.5 to 13 mg l(-1), showed a linear correlation, Rotating disk experiments were carried out in order to estimate the number of electrons involved on the process. It was observed that for both modified electrodes. dissolved O-2 was reduced to H2O2, in a two-electron process.